Maybe try faking/baiting into an armbar. Perhaps start some tech that is likely to produce an attempted counter that sets partner up for armbar. Ude gatame...partner tries to straighten out...then you slip your arm underneath and try to bridge the elbow...or something like that.

If your school starts your randori in the standing...you could try getting it there. That's been something guys at my dojo have been trying to pull off lately.

What is the best position to you for setting up an armbar? I like side mount honestly but people usually know what im doing.

Your question is so general that the best response is, "Whichever one is available."

In other words, it's situationally dependent. What position can you execute the armbar from that results in the highest success rate? Right now, that's probably your best answer.

The flip side of that is you'll never get good pulling off armbars from other positions unless you practice them.

First, your question reveals you to be a pretty green grappler. Second, drill your favorite armbar at least 20 times every time you train for two months, and I guarantee you'll get better at that variation whether your fellow white belts know it's coming or not.

If you drill any technique thousands of times, you will get very good at that technique. /captain obvious

I'm new to bjj myself, but I find it easy, as do other more experienced practicioners to execute the armband from shoulder jump, as it is relatively easy to switch to triangle/ Oma platta/ shoulder lock of anything goes wrong

Shinya Aoki's famous flying armbar will do the trick as well....Armbar is one of those techniques that should be practiced for many different applications.....its also a move that many fighters are very conscious of and do a lot of practice to avoid so keep that in mind as well...first learn to properly apply the technique then work on your timing and variations....thats one of those techniques to work best when done fast and accurately unlike many throws that are still effective even if a step is compromised